Hi TechInTn,
I appreciate all the troubleshooting steps you have performed. You have already replaced the motherboard and tried different ac adapters as well but issue is still the same. However, if the processor goes bad the system usually beeps.
It seems that the issue is with the power port where the ac adapter connects into the system. if not already tried, you may try replacing the DC-in port of the system.
The part number for the port is PFYC8. You may refer to the image below:
Keep me posted with the results. I will be glad to assist further.